The present invention relates to a method of forming a surface coating layer suitable for cutting tools and anti-wear parts in which a high wear resistance, oxidation resistance and anti-weld characteristic are essential requisites, as well as to a surface-coated hard alloy member produced by the amove-stated method. More particularly, the invention is concerned with a surface-coated cutting tool having superior flank-wear resistance, crater-wear resistance and good adherence between the surface coating layer and the matrix metal, as well as between different layers, which in combination ensures a high cutting performance of the cutting tool, and also concerned with a method of producing such a cutting tool.
Throughout this specification, the term "hard alloy" is used to mean alloys obtained by combining one or more of carbides, nitrides and carbonitride of substances belonging to IV to VI groups of the periodic table by means of one or more of matrix metals such as Co, Ni, W, Mo and Cr, e.g. cemented carbide, cermet and so forth.
A so-called coating tip contains as its major component or base material a cemented carbide which is bonded mainly by cobalt. The base material is coated with one or more coating layers of carbide, nitride or carbonitride of a substance belonging to IV to VI groups of periodic table having a superior wear resistance to the base material, the coating layer or layers usually has a thickness of several microns. This coating tip advantageously exhibits a superior stiffness provided by the base material and a high wear resistance ensured by the surface coating layer. It is well known that this coating tip has a superior cutting performance to that of conventional tool made of cemented carbide.
Recently, however, there is an increasing demand for cutting tools having a long life and further improved cutting performance, as a result of advance and development of processed material, and various tools have been newly developed to cope with this demand. The tendency of switching over from single coating layer to multi-coating layer is an example of this development.
It is well known also that TiC (titanium carbide) coating tip and TiN (titanium nitride) coating tip are most popular among the coating tips heretofore proposed. The TiC coating tip, however, has an inferior crater-wear resistance, although it exhibits a high flank-wear resistance. It is also known that the TiN coating tip, which inherently has a lower hardness than TiC coating tip and, hence, a lower flank-wear resistance than TiC, exhibits a higher chemical stability than TiC which in turn ensures a superior crater-wear resistance. Therefore, it has been proposed to make use of the advantages of these single layers, by suitably combining these layers. For instance, Japanese Patent Laid-open Publication No. 3841/1974 discloses a coating tip having a double coating layer consisting of TiC and TiN layers. This coating tip having double coating layer possesses good flank-wear resistance provided by TiC coating layer and good crater-wear resistance provided by the TiN coating layer.
On the other hand, the cutting of cast article requires a high anti-weld characteristic of the tool. To meet this requirement, the cutting tool has to exhibit a high chemical stability particularly at high temperature. A coating tip having a double coating layer of TiC and Al.sub.2 O.sub.3, formed by coating the TiC layer with aluminum oxide (Al.sub.2 O.sub.3) has been proposed as in Japanese Patent Laid-open Publication No. 42029/1976, to cope with demand. In these coating tip having a double coating layer incorporating layers of carbide, nitride and oxide in laminated state, the coating layers are likely to be delaminated or separated, and unfavourable chipping is liable to occur, due to a poor adherence between laminated layers.
Various countermeasures have been proposed to overcome this problem. For instance, Japanese Laid-open Publication No. 60808/1978 proposes to interpose an intermediate layer of titanium oxycarbide TiC.O between the TiC and Al.sub.2 O.sub.3 layers. Meanwhile, Japanese Patent Laid-open Publication No. 151279/1976 discloses a technique in which, after coating the cemented carbide with carbide, nitride or carbonitride, the cemented carbide with coating layers are subjected to a diffusion treatment and oxidation treatment and, finally, the coating layers are coated with Al.sub.2 O.sub.3. This process is, however, too complicated and involves danger because of the use of hydrogen as the reaction gas, although it can improve the adherence between adjacent layers considerably.
Under these circumstances, the present inventors have made intense study to find out a material which exhibits a higher flank-wear resistance and crater-wear resistance, as well as higher adherence of the coating layers, than conventional commercially available coating tips.
As a result of the study, the inventors have reached a conclusion that the following items (1) to (4) are essential requisites for the coating material.
(1) To have a high hardness at high temperature.
(2) To have a coefficient of thermal expansion as close as possible to that of the substrate of tip.
(3) To have a high chemical characteristic, i.e. to have a high level of formation free energy.
(4) The cutting edge of the tip well reaches a temperature as high as 800.degree. C. Therefore, the formed oxide of the coating film must be stable and provides a good lubrication between the cutting edge and the cut material. It is also necessary that the oxide effectively prevents diffusion of cut material into the substrate.
The above requisites (1), (2) and (3) are satisfied optimumly by a coating layer of hafnium compound such as HfC, HfN, HfC.N. For instance, the HfC coating layer exhibits a hardness of Hv1000 at 800.degree. C., which is twice as large as that (Hv500) exhibited by the TiC coating layer at the same temperature. The HfC layer is superior to TiC coating layer also in the requisites (2) and (3).
The requisite (4) above can be most suitably met by coating layer of titanium compound such as TiC, TiN and TiC.N. The formed oxide, i.e. titanium oxide TiO.sub.2 has a minute structure and, therefore, effectively prevents the cut material from being diffused into the base material of cemented carbide. Namely, it has been confirmed that the coating layer of titanium oxide provides an effective barrier.
The present inventors considered, therefore, to combine the hafnium compound and the titanium compound to obtain a coating tip which possesses both of high flank-wear resistance and crater-wear resistance.
However, at the present stage, almost no study nor research has been made in Japan as to the HfC coating film, because it is extremely difficult to form as compared with the TiC layer. Needless to say, HfC coating tip is not available commercially at all. In the U.S.A., a coating tip having HfC coating film, making use of the technique shown in Japanese Patent Laid-open Publication No. 103848/1974 has been sold since 1971 or so. This technique is to form the HfC film by at first forming the metallic hafnium on the tip substrate by evaporation, gas decomposition, plating and the like method, and then transforming it into HfC by allowing the hafnium to react with carbon such as WC on the tip substrate, by way of a heat treatment.
This reaction is made in accordance with the chemical reaction formula shown below. EQU 2Hf+3(WC-Co).fwdarw.2HfC+W.sub.3 Co.sub.3 C (1)
In this case, since the carbon on the substrate surface moves into the Hf layer, .eta. phase (W.sub.3 Co.sub.3 C) is liable to be formed on the substrate surface. This .eta. phase is extremely fragile and deteriorates the adherence of the film. In addition, it is extremely difficult to control the thickness of the HfC coating film, and a large fluctuation of the thickness is inevitable. For this reason, the advantage inherent in the HfC coating film cannot be effectively utilized, and the cutting tool of this type can have only little practical utility.
Hitherto, it has been attempted to form the HfC film by chemical vapor deposition (CVD) method, making use of methane, hydrogen and hafnium tetrachloride. This process is carried out by heating the base material up to a temperature above 1500.degree. C. Therefore, this coating adversely affects the base material and deteriorates the performance of the coating tip, resulting in a limited use or application of the material. In addition, it is difficult to effect a hydrolysis of the halide of hafnium such as HfCl.sub.4 as a decomposition material, so that the work is inconveniently rendered in-efficient.
To overcome this problem, Japanese Patent Laid-open Publication No. 36585/1977 discloses to form a hafnium coating film. According to this process, at first a low grade hafnium halide (HfCl.sub.3, HfCl.sub.2, HfCl) is formed by making the hafnium react with hydrogen chloride and chloride gas. Then, hydrocarbon gas and a carrier gas such as H.sub.2, He, Ar, Kr are introduced to effect a thermal decomposition at 900.degree. to 1300.degree. C., thereby to form HfC. In this method, as stated above, a low grade halide is used as the starting material, so that the high grade halide which has been generated is reduced by being passed through hafnium, or is reduced by making use of H.sub.2. This process, therefore, involves a highly complicated reaction system and, in addition, suffers an inferior efficiency of work and danger due to the use of a carrier gas such as H.sub.2. Further, the HfC coating film is not formed at uniform adherence.
Under these circumstances, the present inventors have developed and established an improved method of making a coating film of hafnium compound which ensures, as will be described later, a minute structure and good adherence of the coating film. According to the invention, a hafnium compound and a titanium compound are suitably combined with each other by making use of this method, so as to provide a surface coated hard alloy material which exhibits, not to mention to the advantages inherent in the hafnium and titanium compounds, a superior performance which can never be presented by the hafnium nor titanium compound solely.
The present inventors have made various studies and researched to put the surface coated hard alloy material coated with hafnium compound into practical use.
As a result, the inventors have found that the hafnium compound is formed easily by at first effecting a reaction of hafnium with halogen to form mainly a high grade halide and then effecting a reaction with a reaction gas such as hydrocarbon supplied into the same reaction system, under the presence of a glow discharge. It has been confirmed also that a titanium compound is formed by the same process allowing a reaction of the titanium halide with the reaction gas such as hydrocarbon. Also, the present inventors have found that the solid solution of a hafnium compound and a titanium compound can be formed by making a simultaneous use of a hafnium halide and a titanium halide as the source material gas and allowing a reaction of these halides with a reaction gas such as hydrocarbon supplied thereto, under the presence of a glow discharge.
The inventors have found that a solid solution of hafnium compound and a titanium compound of a certain composition range provides a superior wear resistance to the single coating layer of hafnium compound and titanium compound. In view of this fact, the inventors have succeeded in developing a coating tip having a double coating layer of a solid solution layer composed of a hafnium compound and a titanium compound, and a layer of a titanium compound formed on this layer of solid solution.
The inventors have confirmed that this coating tip exhibits a superior cutting performance.
The formation of each coating layer basically relies upon chemical vapor deposition method (CVD method) which has been broadly used. However, the present inventors have further improved this method and worked out a method in which the coating reaction is carried out under a reduced pressure in a glow discharge including a plasma discharge. The method for forming the coating layer of the invention, therefore, is more appropriately expressed by a term LPPCVD (Low Pressure Plasma Chemical Vapor Deposition) method. Since this LPPCVD method is carried out under reduced pressure, the coating film formed by this method has a uniform and minute structure. Further, since the reaction is made in a glow discharge, the coating reaction takes place more easily and the formation rate is increased advantageously.
These and other objects, as well as advantageous features of the invention will become more clear from the following description of the preferred embodiments taken in conjunction with the accompanying drawings.